werecorpse
Adventurer
I've never run it in forgotten realms but when I ran it in greyhawk I also included Vesicant. Snap.
I don't know the geopolitical set up of the forgotten realms so I can't be of any specific help.
Positioning wise I don't think the slavers should be located in a country where slavery is legal. In greyhawk the slave ships prey on the Wild Coast which is a region of free and independent coastal towns. They create fear with their rising dominance and slave taking and piratical raids. They then travel with their slave cargo to Highport where they sell most of their slaves to slave traders, so it's in Highport that the characters meet ships from Thay etc but these people aren't the slavelords themselves (although it would be appropriate to have one sit on the slavelords council to replace the scarlet brotherhood representative). The slavelords are middlemen, a rising crime group.
A1 is set in Highport, a ruined coastal city that was at one stage overrun by conquering humanoids who still somewhat control the town together with a range of evil types (orcs in gh but not relevant) and it is now an outlaw town, a hive of scum and villainy! Where the slave trade is kept secret, not because the law would step in but because the slave lords are secretive about their business. A3&A4 is in a mountainous region and A2 is located in the wildness between A1 & A3.
I don't know Forgotten realms well enough to answer your question but IMO keep A1 in an uncivilised area part way between the source of slaves and the slave owners - it's the outlaw market town, with A3&4 in some nearby mountains and A2 part way between the two locations.
